Well, one of the biggest advantages of this system is that everybody reads the same chart! If you write a bunch of style notes and bar counts, chances are only you would be able to read them. I can write a NNS chart, pass it out to a band and make it through a song.
I noticed someone had said that the numbers don't mean much to drummers. While it's true that we don't play chords, don't you want to know what the rest of the band is playing? For me, knowing what chord the band is going to on the bridge determines what I'm going to play. The other advantage is if you can follow along with the numbers, you never have to count. All you have to do is listen to the chords as the go by to keep you place.
My final reason for wanting to have the numbers in my chart is, as musicial director, it is my responsibility to know what the rest of the band is doing. It is also very helpful to "speak thier language." They hate is when you ask them to take the song from 4 bars after that big fill..... :-)